The present invention relates to a rubber composition for side reinforcement excellent in run flat performance and a run flat tire using the same.
At present, a run flat tire having a side reinforcing layer with high hardness arranged at the inside of a side wall part is practically used and a vehicle can run for a certain distance even if an air pressure is lost by puncture. Thereby, there is no necessity to always have a spare tire ready, and it can be expected that the weight of the whole vehicle is decreased.
However, there are limitations on speed and distance for run flat running with a puncture of a run flat tire. Therefore, a further improvement in durability of the run flat tire is desired.
As effective means for improving durability of the run flat tire, an example is a method of suppressing deformation by thickening reinforcing rubber and preventing fracture by the deformation. However, since the tire weight is increased, this goes against weight decrease which is the initial purpose of the run flat tire.
Further, as effective means for improving durability of the run flat tire, am example is a method of increasing a weight of a filler for reinforcement such as carbon black and enhancing the hardness of a reinforcing rubber by compounding the filler to suppress deformation. However, since the load in the steps of kneading, extrusion and the like is large, and the exothermic properties after vulcanization are increased, improvement in durability of the run flat tire cannot be largely expected.
Further, as effective means for improving durability of the run flat tire, an example is a method of increasing a vulcanization density by using a large amount of a vulcanizer and a vulcanization accelerator without increasing the amount of carbon black and suppressing deformation and heat generation (for example, the JP-A-2002-155169). However, elongation of the rubber is lessened, and strength at break is lowered.
On the other hand, it is proposed that improvement in air permeability resistance and appearance is developed by compounding a rubber for a sidewall of a pneumatic tire with natural lamellar mineral such as micas (for example, JP-A-2003-292685 and JP-A-11-348513). However, since such rubber is required to have bending resistance performance, the hardness thereof is low. Therefore, there has been a problem that such rubber has inadequate hardness for being used as a rubber for reinforcing the side of the run flat tire to sustain the load at a low inner pressure.